These studies are concerned with the role of the Fc receptor in activation of human platelets by immune complexes composed of Histoplasma capsulatum yeasts and IgG. Several types of complexes utilized in these studies are made of Protein A-bearing staphylococci and IgG. We plan to investigate the mechanism of binding of IgG to H. capsulatum and to human platelets. Furthermore, we plan to examine what type of changes are required in human platelets in order to initiate the functioning of the Fc receptor. Alternatively, we plan to address ourselves to the problem of regulation of the Fc receptor by changes in cAMP within platelets.